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How Domain Name Server (DNS) Works?
Domain name Servers (DNS) are an important but invisible part of the
internet, and form one of the largest databases on it. Each machine
on an internet is assigned a unique address, called an IP address,
which is 32 bit number and is expressed as 4 octets. The method user
to represent these IP addresses is known as dotted decimal
Notation". A typical address looks like this: 199.249.150.4
It is very difficult to keep in mind the IP addresses of all the
websites we visit daily, because it's not easy to remember strings
of numbers. However, we do remember words. This is where domain
names come into the picture. If you want to connect to a particular
site, you need to know its IP address but do need to know its URL.
The DNS gets the mappings of the IP addresses and the corresponding
names.
Names and numbers
DNS converts the machine names (such as www.xyz.com) to IP addresses
(such as 199.249.150.9). Basically, it translates from a name to an
address and from an address to a name.
The mapping from the IP address to the machine name is called
reverse mapping .when you type http://www.xyz.com into your browser,
the browser first needs to get the IP address of www.xyz.com. The
machine uses a directory service to look up IP addresses and this
service is called DNS. When you type www.xyz.com your machines
firsts contacts a DNS server, asking it to find the IP address for
www.xyz.com. This DNS server might then contact other DNS servers on
the internet. DNS is therefore is considered as the global network
of servers. The great advantage of DNS is that no organization is
responsible for updating it. It is what is known as distributed
database.
The three letter codes
A DNS server is just a computer that's running the DNS software. The
most popular DNS software is BIND (Berkeley Internet Name Domain)
DNS is hierarchical, tree-structured system. The top is donated
by'.'. And is known as the root of the system. Below the root there
are seven immediate sub domain nodes and these are 'com', 'org', 'gov',
'mil', 'net', 'edu', 'Int', etc.
DNS consists of two components
* Name server
* Resolver
Nameserver:
This performs the task of looking up the names. Usually, there is
one name server for a cluster of machines. If the name server does
not contain the requested information, it will contact another name
server. But it is not required for every server to know how to
contact every other server. Every name server will know how to
contact the root name server, and this in turn will know the
location of every authoritative name server for all the second level
domains.
Resolver:
This runs on a client machine to initiate DNS lookups. It contains a
list of nameservers to use. As we have read, the function of each of
these nameservers is to resolve name queries. There are three types
of nameservers-primary nameserver, secondary nameserver, and caching
nameserver. The secondary nameservers are configured for backup
purposes. Caching nameservers only resolve name queries but do not
maintain any DNS database files. It is important to note here that
any change to primary nameservers needs to be propagated to
secondary nameservers. This is because primary nameservers own the
database records. The changes are propagated via a 'zone transfer'.
HOW "CACHING"WORKS
DNS uses principle of 'caching' for its operation. When a
nameservers receives Information about a mapping, it caches this
information .further queries for the same mapping will use this
cached result, thereby reducing the search cost. The nameservers
don't cache forever. The caching has a component called time to live
(TTL) and the TTL determines how long a server will cache a piece of
information. So when the nameservers caches receive an IP address,
it receives the TTL with it. The nameserver caches the IP address
for the period of time then discards it.
When a process needs to determine an IP address given a DNS address,
it calls upon the local host to resolve the address. This can be
done in variety of ways:
Table look up. On UNIX hosts, the table is /etc/hosts.
The process communicates with a local nameservers. This is named on
a UNIX system.
By sending a massage to the remote system that is identified from
the information in the file/etc/resolve.conf.
When a nameserver receives a query for a domain that is does not
serve, it may send back a referral to the client by specifying
better nameservers. Typically operate in the recursive manner
wherein any DNS server passes requests it cannot handle to higher
level server and so on, until either the request can be handled or
until the root of the DNS name space is reached.
The nameservers contain pointers to other nameserver with the help
of which it is possible to traverse the entire domain naming
hierarchy. A host with the initial nameserver addresses has to be
configured. After this, it is able to use DNS protocols to locate
the nameserver responsible for any part or the DNS naming hierarchy.
Thus when a nameserver receives a request, it can do one of the
following:
It can answer the request with an IP address. This method is called
iterative. In this, the client simply asks the server to resolve a
domain name. The server accesses its database, finds its IP address
and sends that back. If the server does not find the address, it
sends back an error ;DNS not found'). Contact another nameserver and
try to find the IP address for the requested name. Send back a
referral to the client specifying the IP address of better
nameservers.
A popular user interface, called 'nslookup'is available on the UNIX
system. With this, you can perform any DNS function. This program
also displays the result to the user. Using is nslookup, you can
obtain a listing of all the hosts in a zone. In order to do this,
you first need to identify the nameserver for the zone.
The threats that are associated with the DNS are due to the lack of
integrity and authenticity checking of the data held within the DNS.
Also, other protocols can use host names as an access control
mechanism. The internet engineering task force (IETF) has come up
with DNS security (DNSSEC) extensions to DNS protocol. The main
objective to DNSSEC is to provide authentication and integrity to
the DNS. These are provided through the use of cryptographic'.
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